1. Field of the Invention
The present invention relates to a display device used in a cellular telephone, a portable small television, a portable game player, a personal digital assistant (PDA), a small size laptop computer, and the like. In particular, the present invention relates to an illuminating unit called a back light or a front light, and to a display device that uses the illuminating unit.
2. Description of the Related Art
Light emitting diodes (hereinafter called LEDs) are often used as light sources having good turn-on characteristics, with little generated heat or noise, in display devices employed in electronic equipment. In general, a transmitting type color liquid crystal display device displays color images by illuminating liquid crystal display elements provided with a red (R), green (G), or blue (B) filter using a back light illuminating unit that uses a white color LED light source. However, ideal white color LED light sources capable of emitting white light do not exist, and devices have been contrived in order to achieve very natural color representation.
A method disclosed in JP 2002-57376 A, for example, is known as a typical recent method. This method is one in which blue color light emitted from a blue color LED light source provided with a fluorescent material excites the fluorescent material, thus obtaining light from green color light to red color light according to wavelength conversion. The light obtained by wavelength conversion is added to the blue color light, thus obtaining quasi-white color light. At the same time, red color light emitted from a red color LED light source is further added to the quasi-white color light, thus obtaining nearly ideal white color light. The white light is guided to a light guide plate, and illuminates liquid crystal display elements. Using this method, the nearly ideal white color light can be obtained simply by only performing output adjustments on the red color LED light source. A liquid crystal display device capable of clear color display can thus be achieved. It is necessary to mix the quasi-white color light and the red color light very efficiently, without color shifting, in order to obtain the nearly ideal white color light. However, a method may be used for mixing in which the blue color LED light source, which is provided with the fluorescent material, and the red color LED light source are disposed at both ends of a light pipe as a first LED light source, which is provided with the fluorescent material, and a second LED light source, respectively. The quasi-white color light and the red color light may be uniformly mixed within the light pipe.
On the other hand, with an illuminating unit such as a back light or a front light, a light source is disposed opposing an end surface of a light guide plate so that light emitted from the light source becomes incident from one end surface of the light guide plate. An illuminating unit having this type of light source arrangement is called a side light type illuminating unit. The side light type illuminating unit having LEDs thus disposed is easier to make smaller and thinner, and has a high light utilization efficiency. A plane emission configuration (refer to JP 3301752 B, for example) is known in which a light pipe is disposed on a side surface of a light guide plate, and light emitted from a light source is guided to the light guide plate through the light pipe. As disclosed in JP 3301752 B, a minute prism group may also be provided in a surface of the light guide plate of an illuminating unit that uses a side light type light pipe. The minute prism group is aligned in parallel with a longitudinal direction of the light pipe in order to make light illuminating an object to be illuminated more uniform. In addition, a minute prism group may also be provided to the light pipe itself, thus unifying the intensity distribution of light emitted from the light pipe and at the same time increasing the utilization efficiency of the light.
However, it is necessary to separately align the positions of the light pipe and the light guide plate when performing illumination with a conventional side light type illuminating unit, that is, when performing illumination with a unit in which the light pipe is disposed in a side light style from one side of the light guide plate. Consequently, there is a problem in that it is labor intensive to make adjustments during assembly.
In addition, it is necessary to provide the minute prism group on the side surface of the light pipe at a pitch of about 100 μm in order to guide the illuminating light to the light guide plate side.